Yep, as you say from the third bos on, it's a different game entirely. Wait 'till you see the fifth round - it's set inside some sort of body, with SNES-style warmping scenery, a superb theme, and some savage bullet sprays.Some of the bullets patterns (yes, actual patterns to navigate through in a Mega Drive game!) are really well designed and the music fits the game perfectly.
Music wise, you just gotta let the intro play out. One of the best tunes to ever grace the MD.